Cardiovascular diseases are a leading cause of death that is addressed by the use of advanced medical imaging modality devices involving repetitive patient screening for early disease detection to improve prognosis. Therefore a non-invasive and non-radiating imaging technique such as MRI (magnetic resonance imaging) is useful for diagnosis as it is devoid of risks and long-term side effects to the patient and provides excellent disease specific soft-tissue contrast with specific pulse sequences. Also in MRI, motion can severely degrade image quality and cardiac applications encounter a highly adverse environment due to both cardiac and respiratory motion. Therefore, ultra fast gradient echo imaging techniques such as TrueFISP (Fast Imaging with Steady State Precession) or turboFLASH (Fast Low-Angle Shot) sequences are utilized for cardiac MRI to minimize the adverse effects of motion. However, unlike spin echo sequences, these classes of sequences are sensitive to magnetic field inhomogeneities. Image quality may be compromised by typical flow artifacts or banding artifacts and the artifacts increase with the increase in field strength (e.g., >=3.0T).
The magnetic field inhomogeneity in known clinical MR scanners is typically minimized using both a static shim and a dynamic shim. A static hardware shim is employed after installation of an MRI scanner to compensate for compromised homogeneity of a main magnetic field due to field distortions in an RF and magnetically shielded room at an installation site. Strategically placed shimming plates within the bore of the scanner improve the homogeneity. A dynamic shim comprising generated higher order magnetic field gradients is employed to compensate for dynamic inhomogeneities that are identified by measurement. Insertion of an object or person into a magnet bore distorts a local magnetic field due to susceptibility discontinuities. In particular, in a cardiac study, for example, numerous tissue interfaces, such as interfaces between, lung and myocardium, liver and lung are unavoidable, resulting often in severe inhomogeneities over a region of interest. Measurements are performed of the magnetic field over an anatomical region of interest and corresponding field gradients needed to counter-balance and subsequently homogenize the field are calculated. In order to perform a dynamic shim, a dedicated MRI sequence is used to estimate the main magnetic field. Typically, a multi-echo sequence, such as a DESS (double echo steady state) pulse sequence, is applied in a three dimensional fashion. The accumulation of phase between two echoes is proportional to the main magnetic field at this location and can be used for field estimation.
In known MRI systems, a generated magnetic field for data collection is typically not-synchronized with cardiac motion or with respiratory motion. As a result, magnetic field estimation of a region of interest for cardiac applications (of the heart) is not reliable. Furthermore, MRI signals originating from chemically shifted protons (such as subcutaneous, epicardial and visceral fat) result in additional phase accumulation and compromise the field estimation. A system according to invention principles addresses these deficiencies and related problems.